Networks and Sites: Improve documentation and variable naming in `swi…

…tch_to_blog()` and `restore_current_blog()`.
In `switch_to_blog()`:
* Rename `$blog_id` to `$prev_blog_id` for clarity.
* Rename `$new_blog` to `$new_blog_id` for consistency.
* Pass `$prev_blog_id` as a second parameter to `switch_blog` action, instead of the duplicated `$new_blog_id`. This only clarifies documentation and does not affect functionality, since the values are equal in the context where the DocBlock is located.
In `restore_current_blog()`:
* Rename `$blog` to `$new_blog_id` for clarity.
* Rename `$blog_id` to `$prev_blog_id` for clarity.
Props ChriCo, jeremyfelt, SergeyBiryukov.
Fixes #45594.
git-svn-id: https://develop.svn.wordpress.org/trunk@45794 602fd350-edb4-49c9-b593-d223f7449a82